/Engine/Source/Runtime/MeshConversion/Public/MeshDescriptionBuilder.h
Append an arbitrary polygon to the mesh with the given PolygonGroup ID Unique Vertex instances will be created for each polygon-vertex.
[FPolygonID](API\Runtime\MeshDescription\FPolygonID) AppendPolygon ( const [TArray](API\Runtime\Core\Containers\TArray)< [FVertexID](API\Runtime\MeshDescription\FVertexID) > & Vertices, const [FPolygonGroupID](API\Runtime\MeshDescription\FPolygonGroupID) & PolygonGroup )